Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, which are particularly suitable for an image pickup apparatus, such as a broadcasting television camera, a video camera, a digital still camera, and a silver-halide film camera.
Description of the Related Art
In recent years, a zoom lens having a large aperture ratio, a high zoom ratio, and high optical performance has been demanded for an image pickup apparatus. As the zoom lens having the large aperture ratio and the high zoom ratio, there has been known a positive-lead type zoom lens, in which a lens unit having a positive refractive power is arranged closest to an object side (Japanese Patent Application Laid-Open No. H11-38321 and Japanese Patent Application Laid-Open No. 2011-107693).
In Japanese Patent Application Laid-Open No. H11-38321, there is disclosed a four-unit zoom lens including, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit for zooming, which has a negative refractive power, a third lens unit having a negative refractive power, which is configured to correct image plane variation accompanying zooming, and a fourth lens unit for imaging, which has a positive refractive power. In the zoom lens disclosed in Japanese Patent Application Laid-Open No. H11-38321, during zooming from a wide angle end to a telephoto end, the second lens unit moves to the image side, and the third lens unit moves along a locus convex to the object side. Through appropriate settings of the lens configurations of the respective lens units, the disclosed zoom lens has a wide angle of field, a high zoom ratio, and high optical performance over the entire zoom range, and is thus suitable for a television camera.
In Japanese Patent Application Laid-Open No. 2011-107693, there is disclosed a five-unit zoom lens including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a negative refractive power, a fourth lens unit having a positive refractive power, and a fifth lens unit having a positive refractive power, which is configured not to move for zooming. In this five-unit zoom lens, a zooming portion constructed by three movable lens units, namely, the second lens unit having the negative refractive power, the third lens unit having the negative refractive power, and the fourth lens unit having the positive refractive power, performs zooming and corrects image plane variation accompanying the zooming.
It is relatively easy for the above-mentioned four-unit and five-unit zoom lenses of positive lead type to achieve a high zoom ratio. As the zoom ratio becomes higher, however, a larger variation in various aberrations occurs during zooming, and it becomes more difficult to obtain high optical performance over the entire zoom range. In particular, the variation in various aberrations accompanying zooming is increased due to the third lens unit having the negative refractive power. For example, in a range from the wide angle end to an intermediate zoom position, a variation in spherical aberration is increased, and further, a variation in spherochromatic aberration caused by a wavelength difference is increased.
Thus, in the above-mentioned four-unit and five-unit zoom lenses, in order to obtain high optical performance while achieving a wide angle of field and a high zoom ratio, it is important to appropriately set the lens configuration of the third lens unit. When the lens configuration of the third lens unit is inappropriate, variations in spherical aberration and curvature of field are increased in the zoom range from the wide angle end to the intermediate zoom position, and it is difficult to obtain high optical performance in the entire zoom range with a high zoom ratio.